Katanas, Wakizashis, Tantos etc, all traditional Japanese bladed weapons share the common characteristic of being able to be broken down into components. the blade and tang, tsuba, handle. And the blade can be transported in a Shirasaya. Owners must register them under the "Gun and Sword Law".
In contrast, a "Tanto" knife, is a knife with a Tanto styled blade. It can not be taken apart. This puts it into the "Knife" category.
This came about as a knee-jerk reaction to a knife incident in Tokyo in 2008.
https://en.wikipedia.org/wiki/Akihabara_massacre
The mass media hyped the danger of knives "designed for use against people". The result was legislation that made all daggers with double edges and knives with "Tanto" styled blades illegal to possess.
As a side note, the dagger that was used in the crime was some cheap no-name Chinese made knife. There was considerable dissatisfaction among the knifemakers in Seki when this happened.